Recently, a studless tire has come into wide use, instead of spike tires, tire dust pollution problems.
The studless tires are usually provided with a block type tread pattern for the excellent road grip performance on snowy and icy roads. In the tread portion of such tire, rubber compounds which maintain flexibility at a low temperature are used in order to further improve the road grip performance on the mainly icy roads. Accordingly, due to the flexible tread rubber, tread blocks are greatly deformed by braking and driving forces, and as a result, the conventional studless tires suffer from uneven wear problems as shown in FIG. 7(b) (hereinafter heel/toe wear) when running on dry or wet roads whose frictional coefficient is relatively high.
FIG. 7(a) shows a state of a tread block (b) which is subjected to a braking force F. As the tire rotates, the heel (c) of the block first contacts with the road surface, and the heel portion is first compressed. Thereafter, the toe (d) contacts with road surface, and then due to the difference in speed between the road surface and the inside of the block, a shearing force towards the heel portion (c) is produced. This shearing force increases as the tire rotates and finally lifts the heel of the block from the road surface to decrease the ground pressure. On the other hand, in the toe portion (d), the ground pressure increases. Therefore, when the block is detached from the road surface, the toe portion (d) is subjected to a maximum ground pressure. Thus, a large wear energy concentrates on the toe portion (d) and the amount of wear in this portion becomes larger than the heel portion (c). Contrary to the above, when a driving force acts on the block, the above-explained conditions are reversed, and therefore wear becomes larger in the heel (c) than the toe (d). Thus, heel-toe wear problems arise.